1. Field of the Invention
The present invention relates generally to the production of liquid oxygen and nitrogen through cryogenic distillation of air and, in particular, to a system utilizing a compact, highly efficient, rapid pressure-swing adsorber (PSA).
2. Description of the Related Art
The industrial and commercial uses of nitrogen and oxygen have created tremendous demands for pure oxygen and nitrogen in both liquid and gaseous phases. These demands are primarily met through large-scale stationary production facilities. Unfortunately, these facilities are located a substantial distance from the end user, necessitating the transportation of large quantities of liquid oxygen and nitrogen over substantial distances. For example, mobile medical facilities for emergency response bureaus require large mounts of liquid oxygen at remote locations. As liquid oxygen is highly explosive, and both liquid oxygen and liquid nitrogen must be kept under heavy pressure at extremely low temperatures, the transportation process is both dangerous and expensive.
Oxygen and nitrogen of high purity may be obtained through cryogenic distillation of ambient air. For effective distillation, the ambient air is filtered prior to the distillation process. In particular, chemicals, H.sub.2 O, and CO.sub.2 must be removed to a concentration of less than 1 part per million (ppm) prior to the airstream entering the distillation columns. One such portable liquid oxygen/liquid nitrogen generating system is disclosed in the inventor's U.S. Pat. No. 4,957,523.
The process of adsorption is the assimilation of gas, vapor, or dissolved matter by the surface of a solid. Generally, adsorbers comprise an outer containment vessel with adsorbent material, or desiccant, distributed within, through which a fluid being filtered passes. There are many types of adsorbent material, including molecular sieves, activated alumina, silica gel, adsorbent clays, and activated carbon. Within each class of adsorbent there are hundreds of variations, both in chemical composition and granular form. The granular form includes such shapes as spherical beads, pellet extrudates, tablets, and irregular granules. While adsorbents used in industry are extremely rugged, they can be destroyed if either the internal or external stresses encountered in the service environment are excessive.
Currently, there are two general classes of adsorbers: temperature-swing adsorbers (TSAs) and pressure-swing adsorbers (PSAs). Both adsorbers have two stages of operation: one in which the fluid is filtered and the other in which the adsorber is regenerated or purged of the contaminants which have adsorbed into the material. TSA adsorbers have a filtering stage at around 40.degree. F. and must be purged at relatively high temperatures (around 500.degree. F.). TSA adsorbers typically require at least three hours to change from filtration temperature to regeneration temperature, to complete the regeneration and to change back to process temperature (one regeneration cycle). This regeneration cycle permits substantial contamination of the filtration bed. PSAs, on the other hand, have a relatively constant temperature, but filter at a high pressure and purge at a low pressure. Rapid PSAs have been developed with regeneration cycles of between 30 and 90 seconds. Such rapid pressure swings, however, necessitate the use of immobilized adsorbent material to prevent the fluidization or abrasion of the adsorbent beads or grains. Immobilizing the adsorbent material by coating and bonding the beads or grains means that the entire adsorber must be replaced if the adsorbent material becomes overly contaminated.
The inventor's U.S. Pat. No. 4,957,523 discloses the use of a dual-bed, immobilized, rapid PSA unit. The PSA includes two immobilized molecular sieve-type, bonded regenerable packed cylindrical beds. When one of the beds is on-line, processing the inlet airstream, the second bed is off-line being purged and regenerated. The regeneration of the off-line bed allows the invention to operate continuously without shutting down during periods of bed regeneration. Typically, one bed is online for 95 seconds, while the flow stream is filtered. During this 95 seconds the second bed is first depressurized, or dumped, then purged, and then pressurized in preparation for going on-line again. The stresses generated on the adsorbent material because of the rapid pressure swings necessitate the use of immobilized beds.
For processes with two adsorber beds to be continuous, one adsorber bed must be depressurized from the on-stream pressure to the purge pressure, purged of the impurities, and repressurized to the on-stream pressure during the period of time that the other adsorber bed is purifying or separating the feed gas for the process. The "feed gas" is the unfiltered airstream entering the adsorption units. As a general rule of thumb, for the off-stream adsorber bed to be adequately purged, the purge gas must be of a volume at least equal to the volume of feed gas that passes through the adsorber bed, and preferably more than 1.5 times the feed gas on-stream volume. For example, if 100 cubic feet of feed gas were purified during the on-stream period, 100 cubic feet or more of purge gas must pass through the adsorber bed during the off-stream purging period. The gas used for purging the off-stream adsorber bed is usually a portion of the purified gas exiting the on-stream adsorber bed. Since the gas exiting the on-stream adsorber bed is used for the process, the net yield of purified gas is reduced by the amount required for purging the off-stream adsorber bed. With cryogenic air separation processes, sufficient waste gas must be available for purging the off-stream adsorber bed, or additional purge gas must be extracted from the purified air exiting the on-stream adsorber bed. This can make the cryogenic air separation process less efficient than it would have been had the purging gas requirement not been considered.
The time required to depressurize and repressurize the adsorber beds is the function of the on-stream and purge pressures, the volume of the adsorber beds, and the rates of flow into and out of the adsorber beds. If pressurizing and depressurizing occurs too rapidly, the desiccant material may be damaged due to fluidizing or abrasion, with subsequent loss of desiccant and/or fracturing of the desiccant due to the rapid reduction of the pressure on the exterior surfaces of the desiccant before the pressure in the interior of the desiccant is reduced. The time required for depressurizing and repressurizing without damaging the desiccant is usually optimized based upon the physical size of the adsorber beds, and is thus fixed.
For a more portable system, for example if it is desired to shorten the on-stream time so the size of the adsorber beds can be reduced, the off-stream time must also be shortened to match. Since the depressurizing and repressurizing times are fixed, the time shortening period must come from the purging period. Since the purging time must be shortened a disproportionately greater amount than the on-stream time, the purging gas flow rate must be increased in order to maintain an adequate purge gas volume. This results in even less of the purified gas being available for the end process.
There has been a need for a more compact and efficient rapid PSA system utilizing nonimmobilized desiccant material within the adsorber beds.